The present invention relates to a treating method for forming a zinc phosphate coating film on a metal surface. In detail, the invention relates to a treating method for forming a zinc phosphate coating film, which is suitable for electrodeposition coating, especially for cationic electrodeposition coating and which is superior in coating film adhesion and corrosion resistance, especially, warm brine resistance and a property to prevent rust of a scab type (scab corrosion) (hereinafter, this property is referred to a "scab resistance"), on a metal surface having only one kind selected from the group consisting of an iron-based, a zinc-based and an aluminum-based surface or simultaneously having these surfaces in combination of two or more kinds by conversion treatment using an acidic zinc-phosphating solution.
Metal materials have been used in various fields such as automobile bodies and other attachments, building materials, furniture, and the like. Metal easily corrodes owing to oxygen or sulfur oxides in the air; rainwater; seawater; and so forth. Because of this, metal is treated with zinc phosphate as coating pretreatment to prevent corrosion. A zinc phosphate coating film formed by this treatment is required to be superior in adhesion to a metal surface which is a substratum, and also, to be superior in adhesion (secondary adhesion) to a coating film further formed on the zinc phosphate coating film and also, the zinc phosphate coating film is required to have sufficient rust preventability even if it is under corrosive environment. In particular, for an automobile body, the scab resistance, a high order of warm brine resistance and so forth are desired since a metallic material of a base is repeatedly subjected to penetration of salt water or variation of dry and wet atmospheric conditions from a scar of the external plate part.
Recently, there has been increased a case of zinc-phosphating a metallic material having two or more kinds of metal surfaces. For example, to elevate further the corrosion resistance of after-coating in a case of the automobile body, a material plated by zinc or a zinc alloy on a surface of a steel material is used. If a conventional zinc-phosphating treatment is carried out on such a metal surface simultaneously having both of an iron-based surface and a zinc-based surface, the zinc-based surface is inferior in corrosion resistance and secondary adhesion (adhesion after aging test) compared with the iron-based surface.
A zinc phosphate coating film formed on a metal surface does not comprise only zinc phosphate but contains various kinds of metal components besides zinc in order to elevate corrosion resistance. Especially, to obtain a zinc phosphate coating film superior in scab resistance and warm brine resistance, the zinc phosphate coating film has contained nickel as an essential component.
On the other hand, there has been proposed a method for forming a zinc phosphate coating film suitable for cationic electrodeposition coating on a surface of a metal material simultaneously having both of an iron-based and a zinc-based surface by zinc-phosphating the metal material using an acidic zinc-phosphating solution which does not contain nickel as an essential component. In the zinc-phosphating method described in Japanese Official Patent Provisional Publication No. showa 57-152472, an acidic zinc-phosphating solution containing 0.5 to 1.5 g/l of a zinc ion, 5 to 30 g/l of a phosphate ion, 0.6 to 3 g/l of a manganese ion, and a phosphating accelerator as main components is used. In the zinc-phosphating method described in Japanese Official Patent Gazette No. showa 61-36588, an acidic zinc-phosphating solution containing 0.5 to 1.5 g/l of a zinc ion, 5 to 30 g/l of a phosphate ion, 0.6 to 3 g/l of a manganese ion, 0.05 g/l or more of a fluorine ion, and a phosphating accelerator as main components is used in order to form a further superior coating film on a metal surface simultaneously having both of an iron-based and a zinc-based surface and to lower a treating temperature. If necessary, the two kinds of phosphating solutions cited here contains 0.1 to 4 g/l of a nickel ion to elevate further the secondary adhesion and corrosion resistance compared with the case of using a manganese ion alone.
In recent years, environmental regulation has tended to become strict, an amount for use of nickel necessary for forming a zinc phosphate coating film superior in scab resistance and warm brine resistance has begun to be limited, and there is probability that use of nickel will become completely impossible in the future.
In case that the phosphating solutions described in the above-mentioned two publications contain nickel, they make a substratum for cationic electrodeposition coating which is good in the secondary adhesion and superior in the scab resistance and warm brine resistance. However, in case that the solutions do not contain nickel, they make a substratum which is good in the secondary adhesion but inferior in the scab resistance or warm brine resistance. Therefore, for making a zinc phosphate coating film superior in corrosion resistance, use of a phosphating solution containing nickel cannot be helped.